Haemophilus ducreyi causes the genital ulcer disease, chancroid. Chancroid facilitates the acquisition and transmission of HIV. Understanding bacterial factors that contribute to the pathogenesis of H. ducreyi infection is a prerequisite for the development of strategies to prevent chancroid. The Spinola laboratory developed a human infection model that mimics the papular and pustular stages of natural chancroid. As a fellow, I conducted several human challenge trials which prompted my interest in bacterial pathogenesis. The Spinola laboratory used the technique of SCOTS (Selective Capture of Transcribed Sequences) on cDNA made from experimental pustules of human volunteers to identify transcripts of H. ducreyi that may be upregulated in vivo. Eleven of the transcripts identified in SCOTS corresponded to lipoproteins. Six of these (HD0192, HD1170, HD1589, HD1629, HD1808 and HD1829) may localize to the outer membrane based on their predicted structures, encode proteins predicted to be >10kDa (and therefore likely to display surface epitopes), were conserved among 10 H. ducreyi strains of different origins, and had transcripts detected in additional infected samples. Two of the lipoprotein transcripts were also upregulated during infection based on microarray data analysis. We hypothesize that these 6 lipoproteins, which are expressed in vivo, are important in H. ducreyi pathogenesis and that they serve as immune targets that may induce bactericidal or opsonophagocytic antibodies. To test these hypotheses, our specific aims include: to confirm by quantitative RT-PCR whether the lipoprotein transcripts identified in SCOTS are upregulated or constitutively expressed in pustules;to examine whether isogenic mutants in upregulated or constitutively expressed lipoprotein genes are attenuated for pustule formation;to characterize the lipoproteins as immune targets. An important feature of this K08 application is a formal didactic component in microbiology and immunology. Lay summary: H. ducreyi is a germ that causes genital ulcers. In order to cause disease, H. ducreyi requires certain proteins. We seek to answer two questions: Which lipoproteins are important in H. ducreyi's ability to cause disease and can any of them serve as vaccines?